Air conditioner

ABSTRACT

The air conditioner includes: an outdoor unit that includes a compressor, an outdoor heat exchanger, and an outdoor expansion valve; an indoor unit that includes an indoor heat exchanger and an indoor expansion valve; a liquid pipe that connects the outdoor unit to the indoor unit; and a gas pipe that connects the outdoor unit to the indoor unit. One end of the outdoor heat exchanger is coupled to the liquid pipe through the outdoor expansion valve. One end of the indoor heat exchanger is coupled to the liquid pipe through the indoor expansion valve. When a predetermined period of time elapses after the compressor is shut down, the outdoor expansion valve and the indoor expansion valve both close.

CLAIM OF PRIORITY

The present application claims priority from Japanese Patent application serial no. 2017-116727, filed on Jun. 14, 2017, the content of which is hereby incorporated by reference into this application.

BACKGROUND Field of the Invention

The present invention relates to an air conditioner, and more particularly, to a multiple air conditioner that is characterized by expansion valve control exercised after a compressor is shut down.

Description of the Related Art

It is known that a refrigerant distribution in an air conditioner during an operating period is significantly different from that during a shutdown period. FIG. 6 illustrates an example of refrigerant distribution in an air conditioner, and is obtained by roughly dividing the interior of the air conditioner into four portions, namely, a connection liquid pipe, a connection gas pipe, an outdoor unit, and a remaining portion, and comparing the refrigerant distribution during an operating period with the refrigerant distribution during a shutdown period (a period between the beginning of shutdown and a time point at which a predetermined period of time has elapsed after shutdown). The result of comparison indicates that the amount of refrigerant in the connection liquid pipe and the outdoor unit is decreased, and that the amount of refrigerant in the connection gas pipe is increased.

It should be noted that the amount of refrigerant in the connection liquid pipe is significantly changed. In the example of FIG. 6, the amount of refrigerant in the connection liquid pipe during the operating period, which is approximately 60 percent of the total, is decreased to approximately 40 percent during the shutdown period. The reason is that a considerable amount of refrigerant in the connection liquid pipe moves into the connection gas pipe.

SUMMARY OF THE INVENTION

If the refrigerant moves from the connection liquid pipe to the connection gas pie during the shutdown period as indicated in FIG. 6, the air conditioner cannot be started up until the refrigerant in the connection gas pipe moves into the connection liquid pipe so that an appropriate amount of refrigerant accumulates in the connection liquid pipe. That is to say, it takes a considerable amount of time for the air conditioner to start up.

Further, when the refrigerant moves from the connection liquid pipe to the outdoor unit, the refrigerant may accumulate, for example, in the connection gas pipe or in an accumulator. When the air conditioner is to be restarted during such accumulation, there is a high risk of compressing a liquid in the compressor. Thus, it is necessary to avoid the risk of liquid compression by increasing the cubic capacity of the accumulator.

The present invention has been made in view of the above circumstances, and provides an air conditioner that not only suppresses the movement of a refrigerant from a connection liquid pipe during a shutdown period to reduce the time required for a restart and improve comfort and reliability, but also prevents a manufacturing cost from being increased by an increase in the size of an accumulator.

According to an aspect of the present invention, there is provided an air conditioner including an outdoor unit, an indoor unit, a liquid pipe, and a gas pipe. The outdoor unit includes a compressor, an outdoor heat exchanger, and an outdoor expansion valve. The indoor unit includes an indoor heat exchanger and an indoor expansion valve. The liquid pipe connects the outdoor unit to the indoor unit. The gas pipe connects the outdoor unit to the indoor unit. One end of the outdoor heat exchanger is coupled to the liquid pipe through the outdoor expansion valve. One end of the indoor heat exchanger is coupled to the liquid pipe through the indoor expansion valve. When a predetermined period of time elapses after the compressor is shut down, the outdoor expansion valve and the indoor expansion valve both close.

According to another aspect of the present invention, there is provided an air conditioner including an outdoor unit, an indoor unit, a cooling/heating switching unit, a liquid pipe, a high/low pressure gas pipe, a low pressure gas pipe, and a gas pipe. The outdoor unit includes a compressor, an outdoor heat exchanger, and an outdoor expansion valve. The indoor unit includes an indoor heat exchanger and an indoor expansion valve. The cooling/heating switching unit includes a high/low pressure gas pipe switching valve and a low pressure gas pipe switching valve. The liquid pipe connects the outdoor unit to the indoor unit. The high/low pressure gas pipe connects the outdoor unit to the high/low pressure gas pipe switching valve. The low pressure gas pipe connects the outdoor unit to the low pressure gas pipe switching valve. The gas pipe connects the indoor unit to the cooling/heating switching unit. One end of the outdoor heat exchanger is coupled to the liquid pipe through the outdoor expansion valve. One end of the indoor heat exchanger is coupled to the liquid pipe through the indoor expansion valve. When a predetermined period of time elapses after the compressor is shut down, the outdoor expansion valve and the indoor expansion valve both close or the outdoor expansion valve, the high/low pressure gas pipe switching valve, and the low pressure gas pipe switching valve all close.

According to still another aspect of the present invention, there is provided an air conditioner including an outdoor unit, an indoor unit, a cooling/heating switching unit, a high pressure pipe, a low pressure pipe, a gas pipe, and a liquid pipe. The outdoor unit includes a compressor, an outdoor heat exchanger, and an outdoor expansion valve. The indoor unit includes an indoor heat exchanger and an indoor expansion valve. The cooling/heating switching unit includes a gas-liquid separator, a high pressure pipe switching valve, a low pressure pipe switching valve, and a liquid pressure adjustment valve. The high pressure pipe couples the outdoor unit to the cooling/heating switching unit. The low pressure pipe couples the outdoor unit to the cooling/heating switching unit. The gas pipe couples the indoor unit to the cooling/heating switching unit. The liquid pipe couples the indoor unit to the cooling/heating switching unit. One end of the outdoor heat exchanger is coupled to the high pressure pipe through the outdoor expansion valve. One end of the indoor heat exchanger is coupled to the liquid pipe through the indoor expansion valve. When a predetermined period of time elapses after the compressor is shut down, the outdoor expansion valve, the indoor expansion valve, the high pressure pipe switching valve, the low pressure pipe switching valve, and the liquid pressure adjustment valve all close.

The aspects of the present invention suppress the movement of the refrigerant from the connection liquid pipe during a shutdown period. Therefore, a heating or cooling operation at a restart of the air conditioner can be promptly initiated to improve comfort. Further, the possibility of liquid compression in the compressor can be reduced without increasing the size of the accumulator. Consequently, improved reliability can be provided without increasing the manufacturing cost.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be described in detail based on the following figures, in which:

FIG. 1 illustrates related-art heating shutdown state expansion valve control that is exercised in a cooling/heating switching multi-mode;

FIG. 2 illustrates related-art cooling shutdown state expansion valve control that is exercised in the cooling/heating switching multi-mode;

FIG. 3 illustrates shutdown state expansion valve control that is exercised in the cooling/heating switching multi-mode;

FIG. 4 illustrates an example of a refrigerant pressure difference due to the difference in height;

FIG. 5 illustrates an example of a refrigerant pressure difference due to the difference between indoor and outdoor temperatures;

FIG. 6 illustrates exemplary refrigerant distribution during an operating period and during a shutdown period;

FIG. 7 illustrates isochoric variation of a liquid refrigerant;

FIG. 8 illustrates pressure behavior that is exhibited from a related-art operating period to a related-art shutdown period;

FIG. 9 illustrates pressure behavior that is exhibited from an operating period to a shutdown period while shutdown state expansion valve control is exercised;

FIG. 10 illustrates pressure behavior that is exhibited from the operating period to the shutdown period while shutdown state expansion valve control is exercised;

FIG. 11 is an exemplary flowchart illustrating how shutdown state expansion valve control is exercised;

FIG. 12 illustrates related-art shutdown state expansion valve control that is exercised in a cooling/heating simultaneous multi-mode;

FIG. 13 illustrates shutdown state expansion valve control according to a second embodiment that is exercised in the cooling/heating simultaneous multi-mode;

FIG. 14 illustrates shutdown state expansion valve control (cooling/heating switching unit valve open) according to a modification of the second embodiment that is exercised in the cooling/heating simultaneous multi-mode;

FIG. 15 illustrates shutdown state expansion valve control (heating indoor expansion valve open) according to another modification of the second embodiment that is exercised in the cooling/heating simultaneous multi-mode;

FIG. 16 illustrates related-art shutdown state expansion valve control that is exercised in a two-pipe cooling/heating simultaneous multi-mode;

FIG. 17 illustrates shutdown state expansion valve control according to a third embodiment that is exercised in the two-pipe cooling/heating simultaneous multi-mode; and

FIG. 18 illustrates cooling shutdown state expansion valve control according to a fourth embodiment that is exercised when a supercooling circuit is used.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

First Embodiment

First of all, an air conditioner according to a first embodiment of the present invention will be described with reference to FIGS. 1 to 5 and FIGS. 7 to 11.

<Related-Art Heating Shutdown State Expansion Valve Control>

FIG. 1 is a refrigeration cycle system diagram illustrating related-art heating shutdown state expansion valve control that is applied to an air conditioner 100 in a cooling/heating switching multi-mode. The air conditioner 100 shown in FIG. 1 is configured so that an outdoor unit 10 is connected to indoor units 40 (a generic name for the indoor units 40 a, 40 b, 40 c, 40 d) through a liquid main 21 and a gas main 24, and each indoor unit 40 is in a heating shutdown state. FIG. 1 illustrates a configuration that includes one outdoor unit 10 and four indoor units 40. However, the number of such units is not limited to those depicted in FIG. 1.

The indoor unit 40 a includes an indoor heat exchanger 41 a, an indoor expansion valve 42 a, and an indoor heat exchanger fan 49 a. One end of the indoor heat exchanger 41 a communicates with the liquid main 21 through the indoor expansion valve 42 a. Further, an indoor heat exchanger gas temperature sensor 45 a, an indoor heat exchanger liquid temperature sensor 46 a, and an indoor temperature sensor 73 a are installed at illustrated locations. The indoor units 40 b, 40 c, 40 d have the same configuration as the indoor unit 40 a and will not be redundantly described.

The outdoor unit 10 includes a compressor 11, a four-way valve 12, an outdoor heat exchanger fan 13, an outdoor heat exchanger 14, an outdoor expansion valve 15, a compressor check valve 16, and an accumulator 18. One end of the outdoor heat exchanger 14 communicates with the liquid main 21 through the outdoor expansion valve 15.

Further, an outlet pressure sensor 55, an outdoor heat exchanger liquid temperature sensor 50, an outdoor heat exchanger gas temperature sensor 51, a liquid pressure detector 71, and an outside air temperature sensor 72 are installed at illustrated locations.

The flow of a refrigerant during a heating operation period and during a heating shutdown state will now be described. During a heating operation period, a high-temperature, high-pressure gas refrigerant compressed by the compressor 11 is conveyed to the indoor units 40 through the gas main 24.

In the indoor units 40, the gas refrigerant fed into the indoor heat exchanger 41 exchanges heat with indoor air, condenses into a high-pressure, two-phase refrigerant or a high-pressure, supercooled refrigerant, and is conveyed to the outdoor unit 10 through the indoor expansion valve 42 and the liquid main 21.

In the outdoor unit 10, the fed refrigerant is subjected to flow rate adjustment by the outdoor expansion valve 15, which is open to a desired degree of opening, exchanges heat with outdoor air in the outdoor heat exchanger 14, evaporates into a low pressure gas refrigerant, and is conveyed to the compressor 11 through the four-way valve 12 and the accumulator 18 to complete a refrigeration cycle during a heating operation period. During the heating operation period, the liquid main 21 is nearly filled with a liquid refrigerant, and only the gas refrigerant is present in the gas main 24.

When a transition is made here to heating shutdown, the outdoor expansion valve 15 fully closes and the indoor expansion valve 42 opens as illustrated in FIG. 1.

Immediately after the shutdown of the compressor 11, the pressure in the gas main 24 disposed downstream of the compressor 11 decreases. When a certain amount of time elapses, the pressure in the gas main 24 balances with the inlet pressure of the compressor 11. When the balanced pressures decrease below the pressure in the liquid main 21, the liquid refrigerant in the liquid main 21 moves to the gas main 24 through the indoor expansion valve 42 and the indoor heat exchanger 41, thereby moving the refrigerant as illustrated in FIG. 6. Further, for example, the four-way valve 12 and the compressor check valve 16 are not completely sealed. Therefore, the refrigerant in the gas main 24 may move to the accumulator 18 and the outdoor heat exchanger 14. If the refrigerant in the liquid main 21 is dispersed to various locations in the refrigeration cycle as described above, a low operating efficiency persists at the next start until the refrigerant is properly distributed to various locations in the refrigeration cycle. As a result, it takes a considerable amount of time to start a heating operation.

If the refrigerant accumulates in the outdoor heat exchanger 14 after a heating operation is shut down, a refrigerant in a gas-liquid mixed state enters the accumulator 18 at startup. The accumulator 18 separates a liquid refrigerant from a low-pressure refrigerant in a gas-liquid mixed state and conveys a gas refrigerant to the compressor 11 to prevent liquid compression in the compressor 11. However, if a large amount of liquid returns from the outdoor heat exchanger 14 or if the refrigerant accumulates in the accumulator 18, a liquid refrigerant separation function degrades to cause a high risk of compressor liquid compression. To avoid such a risk, it is necessary to increase the cubic capacity of the accumulator 18. Such an increase in the cubic capacity of the accumulator 18 will increase the manufacturing cost.

<Related-Art Cooling Shutdown State Expansion Valve Control>

FIG. 2 is a refrigeration cycle system diagram illustrating related-art cooling shutdown state expansion valve control that is applied to the air conditioner 100 having the same configuration as depicted in FIG. 1. The connection of the four-way valve 12 is different from that depicted in FIG. 1. The flow of a refrigerant during a cooling operation period and during a cooling shutdown state will now be described with reference to FIG. 2. During a cooling operation period, a high-temperature, high-pressure gas refrigerant compressed by the compressor 11 is conveyed to the outdoor heat exchanger 14 through the four-way valve 12. The gas refrigerant fed into the outdoor heat exchanger 14 exchanges heat with outdoor air, condenses into a high-pressure, two-phase refrigerant or a high-pressure, supercooled refrigerant, and is conveyed to the indoor units 40 through the outdoor expansion valve 15 and the liquid main 21. In the indoor units 40, the fed refrigerant is subjected to flow rate adjustment by the indoor expansion valve 42, which is open to a desired degree of opening, exchanges heat with indoor air in the indoor heat exchanger 41, evaporates into a low pressure gas refrigerant, and is conveyed to the compressor 11 through the gas main 24, the four-way valve 12, and the accumulator 18 to complete the refrigeration cycle during a cooling operation period. During the cooling operation period, the liquid main 21 is nearly filled with a liquid refrigerant, and only the gas refrigerant is present in the gas main 24.

When a transition is made here to cooling shutdown, the indoor expansion valve 42 fully closes and the outdoor expansion valve 15 opens as illustrated in FIG. 2. Immediately after the shutdown of the compressor 11, the pressure in the outdoor heat exchanger 14 decreases. When a certain amount of time elapses, the pressure in the outdoor heat exchanger 14 balances with the inlet pressure of the compressor 11. When the balanced pressures decrease below the pressure in the liquid main 21, the refrigerant in the liquid main 21 moves to the outdoor heat exchanger 14 through the outdoor expansion valve 15, thereby moving the refrigerant as illustrated in FIG. 6. Further, for example, the four-way valve 12 and the compressor check valve 16 are not completely sealed. Therefore, the refrigerant in the outdoor heat exchanger 14 may move to the accumulator 18 and the gas main 24. When the refrigerant in the liquid main 21 is dispersed to various locations in the refrigeration cycle as described above, a low cooling efficiency persists at the next start until the refrigerant is properly distributed to various locations in the refrigeration cycle. As a result, it takes a considerable amount of time to start a cooling operation.

If the refrigerant accumulates in the gas main 24 after a cooling operation is shut down, the amount of liquid return at startup increases to degrade the liquid refrigerant separation function in the accumulator 18 and cause a high risk of compressor liquid compression. To avoid such a risk, it is necessary to increase the cubic capacity of the accumulator 18. Such an increase in the cubic capacity of the accumulator 18 will increase the manufacturing cost.

<Shutdown State Expansion Valve Control according to First Embodiment>

FIG. 3 is a refrigeration cycle system diagram illustrating shutdown state expansion valve control according to the first embodiment that is applied to the air conditioner 100 having the same configuration as depicted in FIGS. 1 and 2. FIG. 3 illustrates a situation where the four-way valve 12 is connected for a cooling operation (FIG. 2). However, shutdown state expansion valve control according to the first embodiment can be applied even when the connection is made for a heating operation (FIG. 1).

When a transition is made from cooling operation to cooling shutdown in the present embodiment, all the indoor expansion valves 42 and the outdoor expansion valve 15 close as illustrated in FIG. 3. As described earlier, if the compressor is shut down and the outlet pressure of the compressor 11 is decreased while related-art shutdown state expansion valve control is exercised, the refrigerant is dispersed to various locations in the refrigeration cycle through the open expansion valves. However, when the expansion valves at both ends of the liquid main 21 are closed as described in conjunction with the present embodiment, it is possible to prevent the liquid refrigerant accumulated in the liquid main 21 from moving to some other locations during an operation and expedite the start of a cooling or heating operation at the next startup.

A liquid refrigerant movement caused by the refrigerant pressure difference between the outdoor unit 10 and the indoor units 40 and a relevant countermeasure will now be described with reference to schematic diagrams in FIGS. 4 and 5.

FIG. 4 is a schematic diagram illustrating a cooling operation shutdown state of the air conditioner 100 that is configured by disposing the outdoor unit 10 at a lower place and disposing the indoor units 40 10 m above the outdoor unit 10. FIG. 4 shows an example in which the refrigerant is likely to move to a lower heat exchanger due to the difference in height between the opposite ends of the liquid main 21. FIG. 4 serves also as a schematic diagram illustrating a heating operation shutdown state of the air conditioner configured so that the indoor units 40 are disposed at a lower place with the outdoor unit 10 disposed above the indoor unit 40.

When a lower expansion valve opens after a cooling operation is shut down, the liquid refrigerant moves to a lower heat exchanger under the influence of a liquid column head. If, for example, the difference in height between the outdoor unit 10 and the indoor units 40 is 10 m, the liquid density of the refrigerant is 1000 kg/m³, and the outdoor expansion valve is open, a pressure difference of approximately 0.1 MPa is generated in the outdoor expansion valve 15 disposed at the lower end of the liquid main 21 so that the refrigerant in the liquid main 21 moves downward. To avoid such a downward movement, it is necessary to close a lower expansion valve during a shutdown state as described in conjunction with the present embodiment. More specifically, the outdoor expansion valve 15 needs to be closed in a cooling shutdown state where the outdoor unit 10 is at a lower place and the indoor units 40 are at an upper place, and the indoor expansion valve 42 needs to be closed in a heating shutdown state where the outdoor unit 10 is at an upper place and the indoor units 40 are at a lower place.

When the outdoor unit 10 and the indoor units 40 are installed at different heights as described above, it is possible to prevent the liquid refrigerant from flowing, for example, into a lower heat exchanger by closing a lower expansion valve in an operation shutdown state.

FIG. 5 is a schematic diagram illustrating an operation shutdown state of the air conditioner 100 that is configured by disposing the outdoor unit 10 in a 17° C. atmosphere and disposing the indoor units 40 in a 20° C. atmosphere. FIG. 5 shows an example in which the refrigerant is likely to move to a lower-temperature heat exchanger due to the difference in temperature between the opposite ends of the liquid main 21. FIG. 5 assumes that the outdoor unit 10 and the indoor units 40 are at the same height and that the liquid main 21 is in a horizontal position.

If the indoor atmosphere temperature is different from the outdoor atmosphere temperature during an operation shutdown period, the refrigerant in the air conditioner 100 exchanges heat with air due to natural convection, thereby causing a refrigerant movement. If, for example, the outdoor temperature is 17° C. and the indoor temperature is 20° C., the liquid refrigerant in the indoor heat exchanger 41 evaporates over an extended period of time, thereby condensing the gas refrigerant in the outdoor heat exchanger 14. The liquid refrigerant in the indoor units 40 then gradually accumulates in the outdoor unit 10. As the saturation pressure at 20° C. is 1.45 MPa and the saturation pressure at 17° C. is 1.35 MPa, the saturation pressure difference is 0.1 MPa when the difference shown in FIG. 5 between the indoor and outdoor temperatures is 3° C. This saturation pressure difference is equivalent to a liquid conveyance force required for a height difference of 10 m shown in FIG. 4 and cannot be ignored. Therefore, when the difference between the indoor and outdoor temperatures is greater than a predetermined level, control may be exercised to close a lower-temperature expansion valve in order to prevent the flow of the refrigerant from the liquid main 21.

FIG. 7 is an image of liquid seal phenomenon in which a liquid pipe is filled with a liquid refrigerant. When the temperature is raised after the valves at both ends of the pipe filled with the liquid are sealed, the pressure of the liquid refrigerant rises. If the pressure rises above the maximum allowable pressure for the pipe, the pipe may become damaged to cause the leakage of the refrigerant. Therefore, when both the indoor expansion valve 42 and the outdoor expansion valve 15 at the opposite ends of the liquid main 21 are to be closed as indicated in FIG. 3, it is desirable to ensure that the liquid main 21 is not filled with the liquid. When, for example, the outlet pressure sensor 55 and the indoor heat exchanger liquid temperature sensor 46 detect the degree of supercooling at the outlet of each indoor heat exchanger 41 during a heating operation and the outlet temperature of each indoor unit 40 is saturated, a two-phase refrigerant is conveyed to the liquid main 21 and the liquid main 21 is highly probably not filled with the liquid. Therefore, no problem arises even if the valves at both ends are closed. Further, when, for example, a heat exchanger acting as a condenser is disposed at a lower place due to height difference construction or the pipe of the liquid main 21 is long, it is highly probable that a two-phase refrigerant results due to a pressure decrease at an end of the liquid pipe. Therefore, no problem arises even if the valves at both ends are closed depending on the status of the refrigerant. Whether the two-phase refrigerant is in the liquid main 21 may be estimated from cycle status, liquid pipe temperature, and liquid pipe pressure prevailing immediately before shutdown.

FIG. 8 illustrates pressure behavior that is exhibited when related-art shutdown state expansion valve control is exercised as depicted in FIGS. 1 and 2. When the air conditioner 100 shuts down its operation, the outlet pressure and the inlet pressure gradually balance with each other to achieve equality. In the above instance, a valve at one end of the liquid main 21 is open. Therefore, the liquid pressure is lower than or substantially equal to the outlet pressure. If, in this instance, the outlet pressure is lower than the liquid pressure, the refrigerant in the liquid main 21 moves to another device, thereby causing a natural decrease in pressure. As a result, various problems mentioned in conjunction with FIGS. 1 and 2 arise.

FIG. 9 illustrates pressure behavior that is exhibited when shutdown state expansion valve close control according to the present embodiment is exercised as depicted in FIG. 3. As is the case with FIG. 8, the outlet pressure becomes lower than the liquid pressure a little while after the air conditioner 100 is shut down. In the case of FIG. 9, however, valves at both ends of the liquid main 21 close after the air conditioner 100 is shut down. Therefore, the liquid pressure in the liquid main 21 remains constant.

When the expansion valves at both ends of the liquid main 21 are closed immediately after the air conditioner 100 is shut down, the liquid pressure is maintained high. However, the liquid pressure may rise due to the influence of height-difference-induced liquid head and the influence of an increase in liquid pipe atmosphere temperature, and run a risk of exceeding the maximum allowable pressure for the liquid main 21. Such a risk can be avoided by closing the expansion valves at both ends of the liquid main 21 a little while after shutdown, that is, several minutes after shutdown. This decreases the liquid pressure at the beginning of shutdown and reduces the risk of damaging the liquid main 21 due to a liquid pressure rise.

FIG. 10 illustrates pressure behavior that is exhibited when shutdown state expansion valve close control is exercised. FIG. 10 shows an example in which the elapsed time is longer than indicated in FIG. 9 and the outside air temperature and the liquid refrigerant temperature are increased. In a state other than a liquid-sealed state, the liquid pressure rises in accordance with an increase in daytime temperature due to a pressure equivalent to the saturation pressure. If the liquid pressure of the liquid main 21 in the liquid-sealed state rises, the liquid main 21 may become damaged. Therefore, when a predetermined threshold value is exceeded by the liquid pipe temperature observed by the outdoor heat exchanger liquid temperature sensor 50 or by the liquid pressure observed by the liquid pressure detector 71 attached to the liquid main 21, the pressure in the liquid main 21 may be lowered by temporarily opening a closed expansion valve to let the refrigerant in the liquid main 21 move to another device. The liquid pressure detector 71 may be a pressure sensor that measures a pressure or a pressure switch that generates an output when a predetermined pressure is exceeded. When the pressure sensor is employed to measure the pressure, the expansion valve may be closed again when the pressure is decreased to a predetermined value or lower. When the pressure switch is employed, the expansion valve may be closed again in a predetermined period of time after actuation.

Shutdown state expansion valve control according to the present embodiment will now be described with reference to the flowchart of FIG. 11. FIG. 11 illustrates shutdown state expansion valve control that is exercised after the air conditioner 100 is shut down, and does not illustrate expansion valve control that is exercised while the air conditioner 100 is operating.

First of all, in step S1, a check is performed to determine whether the air conditioner 100 is operating or shut down. If the air conditioner 100 is operating, step S1 is repeated until the air conditioner 100 shuts down.

In step S2, a check is performed to determine whether a predetermined period of time has elapsed after the air conditioner 100 is shut down. If the predetermined period of time has not elapsed yet, processing proceeds to step S5. In step S5, expansion valve control is exercised with a normal shutdown opening, that is, expansion valve control is exercised as indicated in FIG. 1 or 2 so that one of the expansion valves at both ends of the liquid main 21 is closed with the remaining expansion valve open. The reason why the check is performed in step S2 to determine whether the predetermined period of time has elapsed is that a high liquid pressure prevailing during an operation is maintained when shutdown state expansion valve control according to the present embodiment, which is illustrated in FIG. 3, is exercised immediately after shutdown, and that shutdown state expansion valve control according to the present embodiment is preferably exercised after the liquid pressure in the liquid main 21 is decreased to a certain extent due to the elapse of a certain period of time.

If it is determined in step S2 that the predetermined period of time has elapsed, processing proceeds to step S3. In step S3, a check is performed to determine whether the liquid pressure in the liquid main 21 is equal to or lower than a predetermined value. If the liquid pressure is higher than the predetermined value, processing proceeds to step S5. In step S5, expansion valve control is exercised with the normal shutdown opening. A threshold value used here as the predetermined value is determined in consideration of the maximum allowable pressure for the liquid main 21. When the threshold value is set to be lower than the maximum allowable pressure for the liquid main 21, it is possible to avoid damage to the liquid main 21 even if the liquid pressure in the liquid main 21 increases due, for instance, to an increase in the outside air temperature after shutdown state expansion valve control is exercised in accordance with the present embodiment. If, for example, the maximum allowable pressure for the liquid main 21 is 4 MPa, the threshold value may be set to 2 MPa, which is half the maximum allowable pressure for the liquid main 21.

If it is determined in step S3 that the liquid pressure in the liquid main 21 is equal to or lower than the predetermined value, processing proceeds to step S4. In step S4, a check is performed to determine whether the liquid main 21 is in the liquid-sealed state. If the liquid main 21 is in the liquid-sealed state, processing proceeds to step S5 because the liquid pressure in the liquid main 21 may significantly increase as indicated in FIG. 7 due, for instance, to an increase in the outside air temperature after shutdown state expansion valve control is exercised in accordance with the present embodiment. In step S5, expansion valve control is exercised with the normal shutdown opening. Whether the liquid main 21 is in the liquid-sealed state can be determined by checking for a gas refrigerant in the liquid main 21. However, such a determination may be made comprehensively in accordance, for example, with height difference construction, pipe length, the indoor heat exchanger outlet liquid temperature and degree of supercooling during a heating operation, the liquid pipe temperature and degree of supercooling during a cooling operation, and the outside air temperature and liquid refrigerant temperature during a shutdown period.

If it is determined in step S4 that the liquid main 21 is not liquid-sealed, shutdown state expansion valve control according to the present embodiment is exercised as illustrated in FIG. 3. This ensures that when the expansion valves at both ends of the liquid main 21 close, it is possible to prevent the refrigerant in the liquid main 21 from flowing to another element while avoiding damage to the liquid main 21.

If it is necessary to properly handle a liquid pressure increase caused by an increase in the outside air temperature as indicated in FIG. 10 even when shutdown state expansion valve control is initiated in step S6, expansion valve control may be exercised again with the normal shutdown opening by returning to step S5. When the liquid pressure subsequently decreases again, shutdown state expansion valve control may be exercised as indicated in step S6.

According to the present embodiment, which has been described above, it is possible to suppress the movement of the refrigerant from the liquid main to another element while the operation of the air conditioner is shut down. Therefore, a heating or cooling operation can be started more quickly when the air conditioner restarts. This provides improved comfort. Further, the possibility of liquid compression in the compressor can be reduced without increasing the size of the accumulator. Consequently, improved reliability can be provided without increasing the manufacturing cost.

Second Embodiment

An air conditioner 200 according to a second embodiment of the present invention will now be described with reference to FIGS. 12 to 15. Elements common to the first embodiment will not be redundantly described.

<Related-Art Shutdown State Expansion Valve Control>

FIG. 12 is a refrigeration cycle system diagram illustrating related-art shutdown state expansion valve control that is applied to the air conditioner 200 in a cooling/heating simultaneous multi-mode. The air conditioner 200 shown in FIG. 12 is configured by connecting the outdoor unit 10, the indoor units 40 (a generic name for the indoor units 40 a, 40 b, 40 c, 40 d), and cooling/heating switching units 30 (a generic name for the cooling/heating switching units 30 a, 30 b, 30 c, 30 d) disposed between the indoor units 40 and the outdoor unit 10 with the liquid main 21 and gas pipes such as a high/low pressure gas main 26 and a low pressure gas main 27. The indoor units 40 a to 40 d are in a heating high-pressure shutdown state, a heating low-pressure shutdown state, a cooling shutdown state, and a blowing (low pressure shutdown) state, respectively. FIG. 12 illustrates a configuration that includes one outdoor unit 10 and four indoor units 40. However, the number of such units is not limited to those depicted in FIG. 12.

One end of the indoor heat exchangers 41 in the indoor units 40 is connected to the high/low pressure gas main 26 or the low pressure gas main 27 through the cooling/heating switching units 30, and the other end is connected to the liquid main 21 through the indoor expansion valves 42.

The cooling/heating switching units 30 are branch circuits that selectively connect the indoor units 40 to the high/low pressure gas main 26 or the low pressure gas main 27. The cooling/heating switching units 30 includes high/low pressure gas pipe expansion valves 31 (a generic name for the high/low pressure gas pipe expansion valves 31 a, 31 b, 31 c, 31 d) and low pressure gas pipe expansion valves 32 (a generic name for the low pressure gas pipe expansion valves 32 a, 32 b, 32 c, 32 d). Opening and closing of the high/low pressure gas pipe expansion valves 31 and low pressure gas pipe expansion valves 32 is controlled so as to change the direction of a refrigerant flow in the indoor units 40 and switch between an evaporator action and a condenser action of the indoor heat exchangers 41 (a generic name for the indoor heat exchangers 41 a, 41 b, 41 c, 41 d) in coordination with decompression squeezing and opening/closing operations of the indoor expansion valves 42 (a generic name for the indoor expansion valves 42 a, 42 b, 42 c, 42 d).

The outdoor unit 10 includes a compressor 11, a heat exchanger four-way valve 12 a, a high/low pressure gas pipe four-way valve 12 b, an outdoor heat exchanger 14, an outdoor expansion valve 15, and an accumulator 18. One end of the outdoor heat exchanger 14 communicates with the liquid main 21 through the outdoor expansion valve 15. The other end is selectively connected to the outlet and inlet of the compressor 11 by the heat exchanger four-way valve 12 a. The high/low pressure gas main 26 is selectively connected to the outlet and inlet of the compressor 11 by the high/low pressure gas pipe four-way valve 12 b. In FIG. 12, the high/low pressure gas main 26 and the outdoor heat exchanger 14 are both connected to the outlet of the compressor 11. In FIG. 12, however, the high/low pressure gas main 26 and the outdoor heat exchanger 14 may be connected to the inlet of the compressor 11 depending on the operating status and cooling/heating load ratio of the indoor units 40.

The flow of a refrigerant during an operating period and during a shutdown state will now be described. During an operating period, part of a high-temperature, high-pressure gas refrigerant compressed by the compressor 11 is conveyed to the indoor unit 40 a, which is engaged in a heating operation, through the high/low pressure gas pipe four-way valve 12 b, the high/low pressure gas main 26, and the high/low pressure gas pipe expansion valve 31 a of the cooling/heating switching unit 30 a. In the indoor unit 40 a, the gas refrigerant fed into the indoor heat exchanger 41 a exchanges heat with indoor air, condenses into a high-pressure, two-phase refrigerant or a high-pressure, supercooled refrigerant, and is conveyed to the indoor expansion valve 42 a and to the liquid main 21.

The rest of the high-temperature, high-pressure gas refrigerant compressed by the compressor 11 is conveyed to the outdoor heat exchanger 14 through the heat exchanger four-way valve 12 a, exchanges heat with outdoor air, condenses into a high-pressure, two-phase refrigerant or a high-pressure, supercooled refrigerant, and is conveyed to the liquid main 21 through the outdoor expansion valve 15.

Liquid refrigerants are conveyed from the indoor unit 40 a and the outdoor unit 10 to the liquid main 21 and joined together. The joined liquid refrigerant is then forwarded to the indoor unit 40 c, which is engaged in a cooling operation, subjected to flow rate adjustment by the indoor expansion valve 42 c, exchanges heat with indoor air in the indoor heat exchanger 41 c, and evaporates into a low pressure gas refrigerant. Subsequently, the low pressure gas refrigerant is conveyed to the compressor 11 through the cooling/heating switching unit 30 c, the low pressure gas pipe expansion valve 32 c, and the low pressure gas main 27 to complete the refrigeration cycle. During this operation, the liquid main 21 is substantially filled with the liquid refrigerant, and only the gas refrigerant is present in the high/low pressure gas main 26 and in the low pressure gas main 27.

When a transition is now made to shutdown, in the indoor units 40, the indoor expansion valve 42 a in a heating shutdown state is open, the indoor expansion valve 42 b in a continued shutdown state is closed, the indoor expansion valve 42 c in a cooling shutdown state is closed, and the indoor expansion valve 42 d in a blowing state is closed, as illustrated in FIG. 12. Further, in the cooling/heating switching units 30, in accordance with an indoor unit operating mode, the high/low pressure gas pipe expansion valve 31 a of the cooling/heating switching unit 30 a remains open and the low pressure gas pipe expansion valve 32 b remains closed. The high/low pressure gas pipe expansion valves 31 b, 31 c, 31 d of the cooling/heating switching units 30 b, 30 c, 30 d remain closed, and the low pressure gas pipe expansion valves 32 b, 32 c, 32 d remain open. Moreover, in the outdoor unit 10, the outdoor expansion valve 15 remains open.

Immediately after the compressor 11 is shut down, the pressures of the outdoor heat exchanger 14 and high/low pressure gas main 26 disposed downstream of the compressor 11 decrease. When a certain amount of time elapses, these pressures balance with the inlet pressure of the compressor 11. When the balanced pressures decrease below the pressure in the liquid main 21, the liquid refrigerant in the liquid main 21 moves to the outdoor heat exchanger 14 through the outdoor expansion valve 15 and moves to the high/low pressure gas main 26 through the indoor expansion valve 42 a, the indoor heat exchanger 41 a, and the high/low pressure gas pipe expansion valve 31 a. As a result, a refrigerant movement occurs as indicated in FIG. 6. Further, for example, the heat exchanger four-way valve 12 a and the compressor check valve 16 are not completely sealed. Therefore, the refrigerant in the outdoor heat exchanger 14 and the high/low pressure gas main 26 may move to the accumulator 18. When the refrigerant in the liquid main 21 is dispersed to various locations in the refrigeration cycle as described above, a low operating efficiency persists at the next start until the refrigerant is properly distributed to various locations in the refrigeration cycle. As a result, it takes a long time to accomplish startup. Moreover, the amount of liquid return at startup increases to degrade the liquid refrigerant separation function in the accumulator 18 and cause a high risk of compressor liquid compression. To avoid such a risk, it is necessary to increase the cubic capacity of the accumulator 18. Such an increase in the cubic capacity of the accumulator 18 will increase the manufacturing cost.

<Shutdown State Expansion Valve Control According to Second Embodiment>

FIG. 13 is a refrigeration cycle system diagram illustrating shutdown state expansion valve control according to the second embodiment that is applied to the air conditioner 200 having the same configuration as depicted in FIG. 12. As indicated in FIG. 13, shutdown state expansion valve control according to the present embodiment is exercised so that all the expansion valves, namely, all the indoor expansion valves 42, all the high/low pressure gas pipe expansion valves 31, all the low pressure gas pipe expansion valves 32, and the outdoor expansion valve 15, close after the compressor 11 is shut down. This prevents the liquid refrigerant accumulated in the liquid main 21 during an operation from moving to some other place. Thus, in the air conditioner 200 in the cooling/heating simultaneous multi-mode, the second embodiment provides the same advantageous effects as the first embodiment.

FIG. 14 illustrates shutdown state expansion valve control according to a modification of the second embodiment. In this modification, shutdown state expansion valve control is exercised so that all the indoor expansion valves 42 and the outdoor expansion valve 15 close after the compressor 11 is shut down. This prevents the liquid refrigerant accumulated in the liquid main 21 during an operation from moving to some other place. This modification eliminates the necessity of controlling the expansion valves in the cooling/heating switching units 30. Therefore, fabrication is made easier as compared to FIG. 13. This modification allows the refrigerant in the indoor heat exchanger 41 a engaged in a heating operation to move to the high/low pressure gas main 26 through the high/low pressure gas pipe expansion valve 31 a and allows the refrigerant in the indoor heat exchanger 41 c engaged in a cooling operation to move to the low pressure gas main 27 through the low pressure gas pipe expansion valve 32 c. However, the amount of refrigerant movement is limited. Therefore, this modification provides substantially the same advantageous effects as the second embodiment depicted in FIG. 13.

FIG. 15 illustrates shutdown state expansion valve control according to another modification of the second embodiment. In this modification, shutdown state expansion valve control is exercised so that all the high/low pressure gas pipe expansion valves 31, all the low pressure gas pipe expansion valves 32, and the outdoor expansion valve 15 all close after the compressor 11 is shut down. This prevents the liquid refrigerant accumulated in the liquid main 21 during an operation from moving to some other place. This modification eliminates the necessity of controlling the indoor expansion valves 42. Therefore, fabrication is made easier as compared to FIG. 13. This modification also allows the liquid refrigerant in the liquid main 21 to move to the indoor heat exchanger 41 a and the cooling/heating switching unit 30 a through the indoor expansion valve 42 a engaged in a heating operation. However, this modification prevents the liquid refrigerant from moving to some other place. Consequently, this modification provides substantially the same advantageous effects as the second embodiment and its modification depicted in FIGS. 13 and 14.

Third Embodiment

An air conditioner according to a third embodiment of the present invention will now be described with reference to FIGS. 16 and 17. Elements common to the foregoing embodiments will not be redundantly described.

<Related-Art Shutdown State Expansion Valve Control>

FIG. 16 is a refrigeration cycle system diagram illustrating related-art shutdown state expansion valve control that is applied to an air conditioner 300 in a two-pipe cooling/heating simultaneous multi-mode. The air conditioner 300 shown in FIG. 12 is configured by connecting the outdoor unit 10, the indoor units 40 (40 a, 40 b, 40 c, 40 d), and the cooling/heating switching unit 30 disposed between the indoor units 40 and the outdoor unit 10 with a high pressure main 28 and a low pressure main 29. The indoor units 40 are in the heating high-pressure shutdown state, the heating low-pressure shutdown state, the cooling shutdown state, and the blowing (low pressure shutdown) state, respectively. FIG. 16 illustrates a configuration that includes one outdoor unit 10 and four indoor units 40. However, the number of such units is not limited to those depicted in FIG. 16.

The cooling/heating switching unit 30 includes a gas-liquid separator 63, a first expansion valve 64, a second expansion valve 65, high pressure pipe switching valves 61 (a generic name for the high pressure pipe switching valves 61 a, 61 b, 61 c, 61 d), and low pressure pipe switching valves 62 (a generic name for the low pressure pipe switching valves 62 a, 62 b, 62 c, 62 d). One end of the indoor heat exchangers 41 in the indoor units 40 is connected to the high pressure pipe switching valves 61 and the low pressure pipe switching valves 62, and the other end is connected to a lower liquid pipe of the gas-liquid separator 63 through the indoor expansion valves 42. The high pressure pipe switching valves 61 and the low pressure pipe switching valves 62 are solenoid valves.

Further, the cooling/heating switching unit 30 controls the opening and closing of the high pressure pipe switching valves 61 and the low pressure pipe switching valves 62 so as to change the direction of a refrigerant flow in the indoor units 40 and switch between an evaporator action and a condenser action of the indoor heat exchangers 41 in coordination with decompression squeezing and opening/closing operations of the indoor expansion valves 42.

The outdoor unit 10 includes a compressor 11, a four-way valve 12, an outdoor heat exchanger fan 13, an outdoor heat exchanger 14, an outdoor expansion valve 15, a compressor check valve 16, and an accumulator 18. One end of the outdoor heat exchanger 14 communicates with the high pressure main 28 or the low pressure main 29 through the outdoor expansion valve 15. The communication target main varies with the pressure of the outdoor heat exchanger 14. In general, the high pressure main 28 is connected when the pressure is high, and the low pressure main 29 is connected when the pressure is low.

The flow of a refrigerant during an operating period and during a shutdown state will now be described. During an operating period, a high-temperature, high-pressure gas refrigerant compressed by the compressor 11 is conveyed to the outdoor heat exchanger 14 through the four-way valve 12, exchanges heat with outdoor air, condenses into a high-pressure, two-phase refrigerant, and is conveyed to the high pressure main 28 and the cooling/heating switching unit 30 through the outdoor expansion valve 15 and the check valve. The high-pressure, two-phase refrigerant conveyed to the cooling/heating switching unit 30 is separated into a gas refrigerant and a liquid refrigerant by the gas-liquid separator 63.

Some of the high pressure gas refrigerant separated by the gas-liquid separator 63 is conveyed to the indoor unit 40 a, which is engaged in a heating operation, through the high pressure pipe switching valve 61 a, fed into the indoor heat exchanger 41 a to exchange heat with indoor air, and condenses into a liquid refrigerant. The liquid refrigerant passes through the indoor expansion valve 42 a and the check valve, and flows into the lower liquid pipe of the gas-liquid separator 63. The pressure in the lower liquid pipe needs to be lower than the pressure in the gas-liquid separator 63. Therefore, the first expansion valve 64 and the second expansion valve 65 are controlled to adjust the pressure in the lower liquid pipe.

Meanwhile, the liquid refrigerant separated by the gas-liquid separator 63 is conveyed to the liquid pipe through the first expansion valve. The liquid refrigerant conveyed from the indoor unit 40 a and the gas-liquid separator 63 is conveyed to the indoor unit 40 c engaged in a cooling operation, subjected to flow rate adjustment in the indoor expansion valve 42 c, forwarded to the indoor heat exchanger 41 c to exchange heat with indoor air, and evaporates into a low pressure gas refrigerant. The low pressure gas refrigerant is conveyed to the outdoor unit 10 through the low pressure pipe switching valve 62 c and the low pressure main 29.

The low pressure gas refrigerant conveyed to the outdoor unit 10 is conveyed to the compressor 11 through the check valve, the four-way valve 12, and the accumulator 18 to complete a refrigeration cycle. During this operation, a large amount of liquid refrigerant is present in the high pressure main 28 and the lower liquid pipe of the gas-liquid separator 63.

When a transition is now made to shutdown, in the indoor units 40, the indoor expansion valve 42 a in a heating shutdown state is open, the indoor expansion valve 42 b in a continued shutdown state is closed, the indoor expansion valve 42 c in a cooling shutdown state is closed, and the indoor expansion valve 42 d in a blowing state is closed, as illustrated in FIG. 16. Further, in the cooling/heating switching units 30, the first expansion valve 64 is open, the second expansion valve 65 is closed, and the high pressure pipe switching valve 61 a and the low pressure pipe switching valve 62 b are closed. Moreover, in the outdoor unit 10, the outdoor expansion valve 15 remains open.

Immediately after the compressor 11 is shut down, the pressures of the outdoor heat exchanger 14 and high pressure main 28 disposed downstream of the compressor 11 decrease. When a certain amount of time elapses, these pressures balance with the inlet pressure of the compressor 11. For example, the check valve used by the outdoor unit is not completely sealed. Therefore, the refrigerant in the outdoor heat exchanger 14 and the high pressure main 28 may move to the accumulator 18. When, for example, the refrigerant in the high pressure main 28 is dispersed to various locations in the refrigeration cycle as described above, a low operating efficiency persists at the next start until the refrigerant is properly distributed to various locations in the refrigeration cycle. As a result, it takes a long time to accomplish startup. Moreover, the amount of liquid return at startup increases to degrade the liquid refrigerant separation function in the accumulator 18 and cause a high risk of compressor liquid compression. To avoid such a risk, it is necessary to increase the cubic capacity of the accumulator 18. Such an increase in the cubic capacity of the accumulator 18 will increase the manufacturing cost.

<Shutdown State Expansion Valve Control According to Third Embodiment>

FIG. 17 is a refrigeration cycle system diagram illustrating shutdown state expansion valve control according to the third embodiment that is applied to the air conditioner 300 having the same configuration as depicted in FIG. 16. As indicated in FIG. 17, shutdown state expansion valve control according to the present embodiment is exercised so that all valves, namely, all the indoor expansion valves 42, all the high pressure pipe switching valves 61, all the low pressure pipe switching valves 62, the first expansion valve 64, the second expansion valve 65, and the outdoor expansion valve 15, close after the compressor 11 is shut down. This prevents the liquid refrigerant accumulated in the high pressure main 28 and in the lower liquid pipe of the gas-liquid separator 63 during an operation from moving to some other place. Thus, in the air conditioner 300 in the two-pipe cooling/heating simultaneous multi-mode, the third embodiment provides the same advantageous effects as the first embodiment.

Fourth Embodiment

An air conditioner according to a fourth embodiment of the present invention will now be described with reference to FIG. 18. Elements common to the foregoing embodiments will not be redundantly described.

FIG. 18 shows an example in which a supercooling heat exchanger 19 is used during a cooling operation. Some of the high pressure liquid refrigerant condensed during a cooling operation is used to cool the remaining liquid refrigerant to be conveyed to an indoor location. Some of the refrigerant is conveyed from a supercooling expansion valve 20 to the supercooling heat exchanger 19 in order to cooling the remaining liquid refrigerant, and then forwarded to the inlet of the compressor. In this instance, the liquid pipe is filled with the liquid even if the outdoor unit is disposed at a lower place with the indoor units disposed at an upper place due to height difference construction or even if the pressure loss in the liquid pipe is great due to a long piping system.

Further, cooling can be conducted depending on the conditions until a temperature lower than the outside air temperature is reached. Therefore, if the expansion valves before and after the liquid pipe are closed in the above instance, the liquid-sealed state occurs. If the liquid refrigerant temperature is increased by outdoor air, the pressure of the liquid refrigerant may increase.

When the supercooling heat exchanger 19 is used during a cooling operation, the valves should not be carelessly closed. The valves should be closed after the liquid pipe temperature is raised or after a certain amount of refrigerant in the liquid pipe is moved to another device. The behavior of pressure and refrigerant movement may be estimated from the liquid pipe temperature and pressure during a shutdown period.

DESCRIPTION OF REFERENCE NUMERALS

-   100, 200, 300, 400 Air conditioner -   10 Outdoor unit -   11 Compressor -   12 Four-way valve -   12 a Heat exchanger four-way valve -   12 b High/low pressure gas pipe four-way valve -   13 Outdoor heat exchanger fan -   14 Outdoor heat exchanger -   15 Outdoor expansion valve -   16 Compressor check valve -   18 Accumulator -   19 Supercooling heat exchanger -   20 Supercooling expansion valve -   21 Liquid main -   24 Gas main -   26 High/low pressure gas main -   27 Low pressure gas main -   28 High pressure main -   29 Low pressure main -   30, 30 a, 30 b, 30 c, 30 d Cooling/heating switching unit -   31, 31 a, 31 b, 31 c, 31 d High/low pressure gas pipe expansion     valve -   32, 32 a, 32 b, 32 c, 32 d Low pressure gas pipe expansion valve -   40, 40 a, 40 b, 40 c, 40 d Indoor unit -   41, 41 a, 41 b, 41 c, 41 d Indoor heat exchanger -   42, 42 a, 42 b, 42 c, 42 d Indoor expansion valve -   45, 45 a, 45 b, 45 c, 45 d Indoor heat exchanger gas temperature     sensor -   46, 46 a, 46 b, 46 c, 46 d Indoor heat exchanger liquid temperature     sensor -   47 Outlet temperature sensor -   49, 49 a, 49 b, 49 c, 49 d Indoor heat exchanger fan -   50 Outdoor heat exchanger liquid temperature sensor -   51 Outdoor heat exchanger gas temperature sensor -   52 Liquid pipe temperature sensor -   55 Outlet pressure sensor -   56 Inlet pressure sensor -   61, 61 a, 61 b, 61 c, 61 d High pressure pipe switching valve -   62, 62 a, 62 b, 62 c, 62 d Low pressure pipe switching valve -   63 Gas-liquid separator -   64 First expansion valve -   65 Second expansion valve -   71 Liquid pressure detector -   72 Outside air temperature sensor -   73, 73 a, 73 b, 73 c, 73 d Indoor temperature sensor 

1. An air conditioner comprising: an outdoor unit that includes a compressor, an outdoor heat exchanger, and an outdoor expansion valve; an indoor unit that includes an indoor heat exchanger and an indoor expansion valve; a liquid pipe that connects the outdoor unit to the indoor unit; and a gas pipe that connects the outdoor unit to the indoor unit; wherein one end of the outdoor heat exchanger is coupled to the liquid pipe through the outdoor expansion valve; wherein one end of the indoor heat exchanger is coupled to the liquid pipe through the indoor expansion valve; and wherein, when a predetermined period of time elapses after the compressor is shut down, the outdoor expansion valve and the indoor expansion valve both close.
 2. An air conditioner comprising: an outdoor unit that includes a compressor, an outdoor heat exchanger, and an outdoor expansion valve; an indoor unit that includes an indoor heat exchanger and an indoor expansion valve; a cooling/heating switching unit that includes a high/low pressure gas pipe switching valve and a low pressure gas pipe switching valve; a liquid pipe that connects the outdoor unit to the indoor unit; a high/low pressure gas pipe that connects the outdoor unit to the high/low pressure gas pipe switching valve; a low pressure gas pipe that connects the outdoor unit to the low pressure gas pipe switching valve; and a gas pipe that connects the indoor unit to the cooling/heating switching unit; wherein one end of the outdoor heat exchanger is coupled to the liquid pipe through the outdoor expansion valve; wherein one end of the indoor heat exchanger is coupled to the liquid pipe through the indoor expansion valve; and wherein, when a predetermined period of time elapses after the compressor is shut down, the outdoor expansion valve and the indoor expansion valve both close or the outdoor expansion valve, the high/low pressure gas pipe switching valve, and the low pressure gas pipe switching valve all close.
 3. An air conditioner comprising: an outdoor unit that includes a compressor, an outdoor heat exchanger, and an outdoor expansion valve; an indoor unit that includes an indoor heat exchanger and an indoor expansion valve; a cooling/heating switching unit that includes a gas-liquid separator, a high pressure pipe switching valve, a low pressure pipe switching valve, and a liquid pressure adjustment valve; a high pressure pipe that couples the outdoor unit to the cooling/heating switching unit; a low pressure pipe that couples the outdoor unit to the cooling/heating switching unit; a gas pipe that couples the indoor unit to the cooling/heating switching unit; and a liquid pipe that couples the indoor unit to the cooling/heating switching unit; wherein one end of the outdoor heat exchanger is coupled to the high pressure pipe through the outdoor expansion valve; wherein one end of the indoor heat exchanger is coupled to the liquid pipe through the indoor expansion valve; and wherein, when a predetermined period of time elapses after the compressor is shut down, the outdoor expansion valve, the indoor expansion valve, the high pressure pipe switching valve, the low pressure pipe switching valve, and the liquid pressure adjustment valve all close.
 4. The air conditioner according to claim 1, wherein a predetermined valve closes when the liquid pressure in the liquid pipe is equal to or lower than a predetermined value.
 5. The air conditioner according to claim 2, wherein a predetermined valve closes when the liquid pressure in the liquid pipe is equal to or lower than a predetermined value.
 6. The air conditioner according to claim 3, wherein a predetermined valve closes when the liquid pressure in the liquid pipe is equal to or lower than a predetermined value.
 7. The air conditioner according to claim 1, wherein a predetermined valve closes when the liquid pipe is not liquid-sealed.
 8. The air conditioner according to claim 2, wherein a predetermined valve closes when the liquid pipe is not liquid-sealed.
 9. The air conditioner according to claim 3, wherein a predetermined valve closes when the liquid pipe is not liquid-sealed.
 10. The air conditioner according to claim 1, wherein, if the liquid pressure in the liquid pipe increases after a predetermined valve is closed, a certain valve opens.
 11. The air conditioner according to claim 2, wherein, if the liquid pressure in the liquid pipe increases after a predetermined valve is closed, a certain valve opens.
 12. The air conditioner according to claim 3, wherein, if the liquid pressure in the liquid pipe increases after a predetermined valve is closed, a certain valve opens.
 13. The air conditioner according to claim 7, wherein the liquid pressure in the liquid pipe is estimated based on an output from a temperature sensor installed in a refrigeration cycle process, a pressure sensor, or an outside air temperature sensor installed in the outdoor unit.
 14. The air conditioner according to claim 8, wherein the liquid pressure in the liquid pipe is estimated based on an output from a temperature sensor installed in a refrigeration cycle process, a pressure sensor, or an outside air temperature sensor installed in the outdoor unit.
 15. The air conditioner according to claim 9, wherein the liquid pressure in the liquid pipe is estimated based on an output from a temperature sensor installed in a refrigeration cycle process, a pressure sensor, or an outside air temperature sensor installed in the outdoor unit. 